USING HIDROGEOMORPHOLOGY INTERPRETATION TO IMPROVE
RUNOFF THRESHOLD ESTIMATION IN MEDITERRANEAN EPHEMERAL
STREAMS (RAMBLAS)
Camarasa Belmonte, A.M.; López-García, M.J. & Soriano García, J.
Departamento de Geografía, Universidad de Valencia
Avda. Blasco Ibáñez, 28, 46010 Valencia. Tel: +34963844237
ana.camarasa@uv.es, maria.j.lopez@uv.es, jusogar@alumni.uv.es
(Project: CGL2007-65368, Spanish Ministry of Science and Technology and ERDF)
ABSTRACT:
The runoff threshold (P0) is one of the key parameters in the study of catchment
hydrological response. It is fundamental in the analysis of rainfall-runoff conversion processes, for
estimating water budgets and for describing the genesis of hortonian overland flow, as it provides
information on the precipitation losses within the fluvial systems, from the start of the episode until
runoff is produced.
There are several methods for estimating this parameter. In the case of small semiarid and
Mediterranean catchments, complex models are not suitable due to the specific conditions of these
environments as well as the quantity of data required for the models. In these environments,
empirical methods have demonstrated to be more effective. One of the most used models in Spain
is that developed by the US Soil Conservation Service (SCS) in 1972, and modified by Témez
(1978, 1991). This is a simple, easy to apply model, which uses slope values, vegetation, land use
and soil hydrological group data. In spite of the generalized application of the model and its
multiple adaptations, there are still many uncertainties, mainly related to the soil hydrological
groups which, together with land use, are the most sensitive variables in runoff threshold
estimation.
These uncertainties are especially important in small semiarid catchments and
Mediterranean ephemeral streams where soils are generally very heterogeneous, not well
developed, highly dependent on underlying rock and have been heavily exploited. In order to
improve runoff threshold estimation in semiarid environments, some authors have included detailed
information about soils in the model but without improving the results significantly. Others have
assigned the soil hydrological group directly from lithology, without considering soil type.
This paper proposes some modifications to the SCS method related to the assignation of the
soil hydrological group in order to improve runoff threshold estimation in Mediterranean
catchments. In these environments, hydrological behavior is highly determined by the strong
interaction between geomorphological structure, lithology and edaphology. These three variables
are combined at catchment scale, in order to assign the soil hydrological group with the aim of
using hidrogeomorphological interpretation to improve runoff threshold estimation.
The method has been applied, using GIS, to several small catchments located in the
Mediterranean Spanish region. Although the estimation of runoff threshold using a GIS is a process
which can be easily automated, the proposed modification requires the expert geomorphological
knowledge of the cachtment. Runoff thresholds obtained with this method are presented here.
Although the results have to be checked with runoff threshold data obtained by water budget, so far
they are consistent with the expected values for the study area from literature references.